5-HT 1A receptor-mediated inhibition of lateral vestibular nucleus neurons projecting to the abducens nucleus

Abstract

Electrophysiological studies were performed using cats anesthetized with α-chloralose, to elucidate the 5-hydroxytryptamine (5-HT) receptor subtypes involved in the 5-HT-induced inhibition of the lateral vestibular nucleus (LVN) neurons projecting to or through the abducens nucleus. The effects of 5-HT receptor subtype agonists and antagonist were examined in polysynaptic neurons activated by stimulation of the ipsilateral abducens nucleus (IAN) antidromically, since these neurons are sensitive to 5-HT as shown in our previous study. Iontophoretic application of 5-HT and8-hydroxy-2-(di- n-propylamino)tetrain (8-OH-DPAT), a selective 5-HT 1A agonist, inhibited orthodromic spikes elicited by vestibular nerve stimulation in the majority of polysynaptic neurons activated by stimulation of ipsilateral IAN antidromically. There was a good correlation between the effects of 5-HT and 8-OH-DPAT. Iontophoretically applied 5-HT and 8-OH-DPAT also inhibited glutamate-induced firing in these neurons. Simultaneous application of 1-(2-methoxyphenly)-4-[4-(2-phthalimido)butyl]piperazine (NAN-190), a 5-HT 1A agonist/antagonist, significantly antagonized the 8-OH-DPAT-induced inhibition of glutamate-induced firing, although NAN-190 alone also caused weak suppression of glutamate-induced firing. Microiontophoretically applied 1-(3-chlorophenyl)piperazine (mCPP), a 5-HT 1B agonist inhibited the orthodromic spike elicited by vestibular nerve stimulation and glutamate-induced firing in only a small number of the LVN neurons. 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT 2 agonist, rarely affected these neurons. We postulate that postsynaptically located 5-HT 1A receptors are mainly involved in the 5-HT-induced inhibition of polysynaptic neurons projecting in the region of the IAN.